The thing is that we happen to havw pulsars in our vicinity that have a very predictable frequency with which they change their brightness. Now just like the si

The thing is that we happen to havw pulsars in our vicinity that have a very predictable frequency with which they change their brightness. Now just like the siren on a police car will move up in pitch when it moves towards you and down when ot moves away (the Doppler effect) if space compacts and expands one would expect the same to happen on a very small scale.